Perpendicularly magnetized MTJs (p-MTJs) are a major emerging technology for use in embedded magnetic random access memory (MRAM) applications, and standalone MRAM applications. P-MTJ MRAM technology that uses spin-torque (STT-MRAM) for writing of memory bits was described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), and is an increasingly promising candidate for future generations of non-volatile memory to replace embedded flash memory and embedded cache memory (SRAM).
Both MRAM and STT-MRAM have a p-MTJ cell based on a tunneling magnetoresistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin insulating tunnel barrier layer such as MgO. One of the ferromagnetic layers called the pinned layer has a magnetic moment that is fixed in an out-of-plane direction such as the +z direction when the plane of each layer is laid out in the x-ax is and y-ax is directions. The second ferromagnetic layer has an out-of-plane magnetization direction that is free to rotate to either the +z-ax is (parallel or P state) or the −z-ax is (antiparallel or AP state) direction. The difference in resistance between the P state (Rp) and AP state (Rap) is characterized by the equation (Rap-Rp)/Rp also known as DRR or the MR ratio. It is important for p-MTJ cells to have a large MR ratio, preferably higher than 100%, as the MR ratio is directly related to the read margin for the memory bit, or how easy it is to differentiate between the P state and AP state (0 or 1 bits).
Another critical requirement for p-MTJs is thermal stability during 400° C. process temperatures that are typical of back-end-of-line (BEOL) processes when fabricating embedded memory devices in complementary metal-oxide-semiconductor (CMOS) products. A general trend has been to introduce a second metal oxide/free layer (FL) interface similar to the tunnel barrier/FL interface thereby enhancing PMA and Hk within the free layer, and improving thermal stability. Unfortunately, a consequence of employing a Hk enhancing (cap) layer such as MgO on the free layer is the addition of parasitic resistance to the p-MTJ. Equation (1) shows the effect of the cap layer resistance contribution to total p-MTJ resistance while Equation (2) indicates the impact on DRR (MR ratio).
                              DRR          =                                                                      R                  AP                                -                                  R                  P                                                            R                P                                      ⁢                                                  ⁢            where                          ⁢                                  ⁢                              R            AP                    =                                    R              AP              barrier                        +                                          R                AP                cap                            ⁢                                                          ⁢              and                                      ⁢                                  ⁢                              R            P                    =                                    R              P              barrier                        +                          R              P              cap                                      ⁢                                  ⁢                              Since            ⁢                                                  ⁢                          R              AP              cap                                =                      R            P            cap                                              Eq        .                                  ⁢                  (          1          )                                        DRR        =                                                            R                AP                barrier                            +                              R                AP                cap                            -                              (                                                      R                    P                    barrier                                    +                                      R                    P                    cap                                                  )                                                                    R                P                barrier                            +                              R                P                cap                                              =                                                    R                AP                barrier                            -                              R                P                barrier                                                                    R                P                barrier                            +                              R                P                cap                                                                        Eq        .                                  ⁢                  (          2          )                    
In summary, the series resistance (RAPcap and RPcap) caused by the metal oxide cap layer will cause a reduction in DRR, unfortunately reducing the MRAM bit reading margin as well as increasing the bit's writing voltage by adding a series resistance. Therefore, an alternative p-MTJ cell structure is needed that features a RA product preferably less than 5 ohm-μm2 for advanced memory products while achieving an acceptable Hk (PMA) for enhanced thermal stability, and increasing the MR ratio above 100% while minimizing switching voltage (Vc).